Ubiquitous GPS coverage could soon get spotty

GPS satellites

Global Positioning Systems have become ubiquitous, with support for the satellite-based system in everything from cell phones to cars.  The system too has become a vital part of day-to-day activities from coordinating Coast Guard rescues and 911 calls to scheduling intercontinental flights.  But don't take it for granted too long as its ubiquity might go away in the next few years. 

That's because the US Air Force, which is responsible for the GPS system, is in the process of modernizing it and may not be able to upgrade the systems before some degradation of services happens, according to a report issued this week by the watchdogs at the Government Accountability Office.

In recent years, the Air Force has struggled to successfully build GPS satellites within cost and schedule goals; it encountered significant technical problems that still threaten its delivery schedule; and it struggled with a different contractor. As a result, the current satellite program has overrun its original cost estimate by about $870 million and the launch of its first satellite has been delayed to November 2009-almost 3 years late, the GAO said.

The GAO said the GPS system has had its share of technical difficulties as well.  For example, last year, during the first phase of thermal vacuum testing (a critical test to determine space-worthiness that subjects the satellite to space-like operating conditions), one transmitter used to send the navigation message to the users failed, the GAO stated.  The program suspended testing in August 2008 to allow time for the contractor to identify the causes of the problem and take corrective actions. The program also had difficulty maintaining the proper propellant fuel-line temperature; this, in addition to power failures on the satellite, delayed final integration testing. In addition, the satellite's reaction wheels, used for pointing accuracy, were redesigned because on-orbit failures on similar reaction wheels were occurring on other satellite programs-this added about $10 million to the program's cost, the GAO stated.

Delays in the launch of the new satellites will increase the risk that the GPS constellation will decrease in size to a level where it will not meet some users' needs, the GAO said. If the GPS constellation falls below the number of satellites required to provide the level of GPS service that the US has committed to providing, some military and civilian operations could be affected. The defense department is evaluating alternative approaches that could mitigate the gap. However, procurement of additional GPS satellites does not appear to be feasible, the GAO said.

Because there are currently 31 operational GPS satellites of various blocks, the near-term probability of maintaining a constellation of at least 24 operational satellites remains well above 95%. However, DOD predicts that over the next several years many of the older satellites in the constellation will reach the end of their operational life faster than they will be replaced, and that the constellation will, in all likelihood, decrease in size, the GAO stated.

Based on the most recent satellite reliability and launch schedule data approved in March 2009, the estimated long-term probability of maintaining a constellation of at least 24 operational satellites falls below 95% during fiscal year 2010 and remains below 95% until the end of fiscal year 2014, at times falling to about 80%. program officials provided no evidence to suggest that the current mean life expectancy for satellites is overly conservative, the GAO stated.

The results of spottier GPS coverage could include the following:

Flights: Intercontinental commercial flights use predicted satellite geometry over their planned navigation route, and may have to delay, cancel or reroute flights.

Emergency calls: Enhanced-911 services, which rely upon GPS to precisely locate callers, could lose accuracy, particularly when operating in "urban canyons" or mountainous terrain.

Defense: The accuracy of precision-guided munitions that rely upon GPS to strike their targets could decrease. To accomplish their mission, military forces would either need to use larger munitions or use more munitions on the same target to achieve the same level of mission success. The risks of

collateral damage could also increase.

Directions: Another important consideration is that both the standard positioning service and precise positioning service performance standards assume that users have unobstructed visibility to nearly the entire sky, an

assumption that does not hold for the large number of users operating in

moderately mountainous terrain, in the "urban canyons" of large cities, or

under forest foliage.

So what's to be done?  Well, the Air Force is looking at lower cost alternatives to current GPS satellites.  In 2007 the Air Force Scientific Advisory Board examined whether small satellites-which can be developed more quickly and at relatively low cost-might help.  The board concluded that small satellites may eventually have operational utility in augmenting GPS capabilities. However the need for an extensive control segment infrastructure to monitor and control these small satellite augmentations, combined with the need to develop, produce, and install user equipment, would make it very challenging to field a near-term small satellite augmentation, the GAO said.

Powering down some components of current GPS satellites is also an option.  The operational life of a GPS satellite tends to be limited by the amount of power that its solar arrays can produce. This power level declines over time as the solar arrays degrade in the space environment until eventually they cannot produce enough power to maintain all of the satellite's subsystems. However, according to Air Force representatives, the effects of this power loss can be mitigated somewhat by actively managing satellite subsystems-shutting them down when not needed- thereby reducing the satellite's overall consumption of power. It would also be possible to significantly reduce the satellite's consumption of power by shutting off a secondary GPS payload. This would buy additional time for the navigation mission of the satellite at the expense of the mission supported by the secondary payload, the GAO stated.

In the end of its report the GAO called for the Secretary of Defense to appoint a single authority to oversee development of GPS space, ground control, and user equipment, to ensure they are synchronized, well executed, and potential disruptions are minimized. For its pat the DOD agreed with the GAO.  We'll se where it goes from here.

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